1. Field of the Invention
The invention relates to a modified molecular sieve for wastewater treatment and a preparation method thereof, and more particularly to a modified molecular sieve with high selectivity to ammonia nitrogen in wastewater and a preparation method thereof.
2. Description of the Related Art
There are various methods in the prior art to remove ammonia nitrogen in wastewater such as biotechnology, air stripping, membrane absorption, and precipitation of magnesium ammonium phosphate (MAP). For now, the precipitation of magnesium ammonium phosphate is the hottest technology in international and domestic studies on ammonia-nitrogen wastewater treatment, where magnesium salt and phosphate are added to the wastewater to cause chemical reaction with the ammonia nitrogen. Afterwards, the magnesium ammonium phosphate precipitation (MgNH4PO4.6H2O) is generated and thus the ammonia nitrogen is removed.Mg2++NH4++PO43−+6H2O→MgNH4PO4.6H2O↓
The foregoing method is simple and easy to operate, but in actual applications, as the wastewater quality and amount fluctuate dramatically, it will affect the addition of the fixed amount of the magnesium salt and phosphate. In other words, if they are added more than the amount of the ammonia nitrogen in wastewater, the chemical substances will be wasted; if they are added less than the amount of the ammonia nitrogen in wastewater, the ammonia-nitrogen wastewater treatment effect will be affected. Consequently, a fixed bed reactor is introduced to process the ammonia nitrogen wastewater, which can effectively solve the magnesium salt and phosphate addition problem. However, when a large amount of wastewater is processed by means of the fixed bed reactor, it will generate a large pressure drop. Therefore, such a method is difficult for practical applications. The above technical difficulty hinders the further study and applications of the MAP precipitation technology.
A molecular sieve is a cubic lattice aluminosilicate compound having a porous skeleton structure formed by silicon aluminum through oxygen bridges. The structure includes a plurality of pore canals with even diameters and empty pores which are arranged orderly and with large internal surface areas. Molecular sieves have the advantages of low fluid resistance, high adsorption rate, large adsorption capacities, strong selectivity, and high mechanical strength. However, since ammonia nitrogen cannot be removed with high selectivity purely by a single molecular sieve, thus a problem difficult to be solved nowadays is how to effectively utilize structural characteristics of molecular sieves to both maintain their advantages (e.g. low fluid resistance, stable performance, etc.) and meanwhile remove ammonia nitrogen with high selectivity. At present, studies and applications on a modified molecular sieve using magnesium compounds and phosphorus compounds to remove ammonia nitrogen in wastewater cannot be found in related documents and are not disclosed in any patent.